Events in stratosphere can affect Earth's entire climate

Events high in the upper atmosphere can cause massive shifts in the behaviour even of deep ocean currents, according to new research.

"It is not new that the stratosphere impacts the troposphere," says Reichler, says Thomas Reichler, senior boffin on the team which discovered the effects. "It also is not new that the troposphere impacts the ocean. But now we actually demonstrated an entire link between the stratosphere, the troposphere and the ocean."

Reichler was working with other scientists both at his own university (Utah) and at the Max-Planck-Institut für Meteorologie in Hamburg.

According to a Utah uni statement announcing the research:

Reichler and colleagues used weather observations and 4,000 years worth of supercomputer simulations of weather to show a surprising association between decade-scale, periodic changes in stratospheric wind patterns known as the polar vortex, and similar rhythmic changes in deep-sea circulation patterns.

The ocean is very important to the climate, acting as it does as a vast storage and transport mechanism for heat. For instance the British climate is comparatively mild (given old Blighty's sub-arctic latitude) due to the fact that the seas around the UK bring heat up from the tropics.

But the oceans have tremendous mass compared even to the relatively dense atmospheric layer that rests upon them (the troposphere). Just the upper ten metres of ocean exert as much pressure on the waters beneath as the entire atmosphere does on the ocean's surface. While it's evident just how much the troposphere can affect the oceans even so - winds not only generate mighty ocean waves and swells, but can on occasion hold back the tides, affect currents etc - such things are mostly seen on the surface. The idea of events up in the wispy stratosphere reaching right down into the ocean deeps is certainly a counterintuitive one on the face of it.

Nonetheless, Reichler and his colleagues say that analysis of past history shows that a stratospheric wind pattern above the Arctic - the polar vortex - appears to exert a powerful influence on the critical heat "conveyor belt" in the north Atlantic.

"We found evidence that what happens in the stratosphere matters for the ocean circulation and therefore for climate," says Reichler.

It appears that current climate forecast models don't allow for this effect, and will need to be adjusted for it as it can produce large, decade-long ups and downs in temperature "separate from climate change", according to Reichler. He and his team write:

Our analyses identify a previously unknown source for decadal climate variability and suggest that simulations of deep layers of the atmosphere and the ocean are needed for realistic predictions of climate.

Thus it could be that with the new stratospheric effect added to climate forecasts, periods of flat temperatures like the one seen over the past decade - or even of some cooling, perhaps - might be forecast accurately, presumably against a general long-term upward trend due to increased atmospheric carbon. We are told:

In the 1980s and 2000s, a series of stratospheric sudden warming events weakened polar vortex winds. During the 1990s, the polar vortex remained strong.

Temperature records showed noticeable warming in the 1990s, in contrast to the 2000s.

Other recent research has also suggested that relatively minor stratospheric events could nonetheless have major climate effects. Researchers at the German GeoForschungsZentrums (GFZ) at Potsdam suggested that such a stratospheric mechanism driven by a solar quiet spell may have caused a 200-year-long cold snap (the "Homeric Minimum") some 2800 years ago. Eminent physicists consider that another such solar quiet period may be imminent, and noted that the most recent such occurrence in the 17th and 18th centuries was accompanied by a so-called "Little Ice Age". Nonetheless, mainstream climatologists who have longed warned of carbon-driven disaster have argued that this would not have a powerful enough effect to significantly counteract carbon-driven warming.

Reichler and his colleagues' new paper is published in Nature Geoscience. ®